Fault recorders are used for detecting and recording faults in electrical power supply networks; said fault recorders sample the network with a frequency that is high compared to the network frequency, so that higher-frequency faults are also detected. Network faults can be detected, for example, by comparing the network voltage sampling values in successive periods. Since, on the one hand, the full signal variation cannot be recorded, since this would require a huge number of irrelevant data to be stored and, on the other hand, mere recording of a fault is in general insufficient for analysis regarding its origin and effects, the previous history a few seconds before and the subsequent variation of current and voltage for a few seconds after their decay are recorded. The sampled values can be made available in a memory or a register for later recording, or they can also be transmitted to a central unit where they are stored and analyzed. Despite limiting the method to faults and the immediate previous and subsequent history, considerable amounts of data must be stored and/or transmitted. For example, if the fault has a duration of 0.25 seconds and the 5-second previous and subsequent history are to be recorded, a recording rate of 80 Kbyte/sec results considering a sampling rate of 5 Khz, eight channels (four for voltages and four for currents), and a digitizing resolution of 16 bits. Therefore 820 Kbytes are required for describing the assumed fault during 10.25 sec. At a transmission rate of 300 Baud, their transmission would require 6 hrs, i.e., an intolerably long time. Even at a higher transmission rate of 9600 Baud, the transmission time would not be reduced to a satisfactory level.
Therefore, European Patent Application 477,690 A1 proposes that the sampling values be subjected to data reduction prior to their storage or transmission. Different data reduction methods are known from the literature, e.g., from K. Riedel's book "Datenreduzierende Bildcodierung" [Image coding for data reduction], 1986, Franzis'-Verlag, Munich.